1. Field of the Invention
The present invention relates to a technique concerning a wavelength locker used for optical communications, and more particularly, to a wavelength locker used in an LD module which requires high-precision wavelength stability for, e.g., Dense Wavelength Division Multiplexing (DWDM) communications.
2. Description of the Related Art
In Wavelength Division Multiplexing (WDM) communications, multiple beams of signal light are transmitted at predetermined signal wavelength intervals. Thus, it is necessary to have adequate isolation from signal light of adjacent wavelengths. As the signal wavelength interval becomes shorter, higher precision control needs to be applied to the output wavelengths of signal-light light sources. In WDM communication systems, wavelength lockers are used, which suppress wavelength fluctuation caused by fluctuation over time or fluctuation of surrounding temperature of laser diode (LD) light sources, and lock the outputted light beams at specified wavelengths with high precision.
With increased data traffic of recent years, high-speed high-capacity communications become fundamental. Development of high-capacity photonic networks using DWDM has been in progress. For an optical transmitting and receiving device used for DWDM, compact pluggable modules conforming to MSA (Multi Source Agreement), such as XFP (10 Gigabit Small Form Factor Pluggable) and SFP (Small Form-Factor Pluggable), have been increasingly developed. TOSA (Transmitter Optical Sub Assembly) LD (Laser Diode) modules to be incorporated in the compact pluggable modules are very compact. In order to support DWDM and tunable LDs, a wavelength locker needs to be incorporated in TOSA. However, it is impossible to incorporate the wavelength locker having a conventional structure in TOSA. Therefore, there is an urgent need to develop amore compact wavelength locker.
FIG. 24 is a diagram showing a structure of a conventional wavelength locker. In the wavelength locker shown in FIG. 24, backward light emitted by an LD serving as a light emitting element (light source) is collimated by a lens and separated by a half mirror. One of beams of the light separated is directly received by a PD (photo diode) 1 serving as a light receiving element. The other one of the beams is received by a PD 2 after passing through an etalon filter. The etalon filter has wavelength-dependent transmission property. Accordingly, it is possible to keep the wavelength of outputted light to be incident on an optical fiber constant by controlling the oscillation wavelength of the LD such that a ratio of light-receiving powers of the PD 1 and the PD 2 becomes constant.
However, with the structure of the conventional wavelength locker, a space optical system becomes large because the half mirror separates light into beams that are perpendicular to each other. Further, since the PD 1 and the PD 2 are perpendicularly aligned, amounting area becomes large. In addition, it is necessary to tilt the etalon filter to prevent light reflected by an end surface of the etalon filter from returning to the LD, leading to enlargement of the mounting area. Consequently, a limitation resides in miniaturization of the wavelength locker, so the conventional wavelength locker cannot be incorporated in TOSA to be mounted to the compact pluggable modules such as XFP.
Note that prior art documents of the present invention include the following Patent documents.
[Patent document 1] JP 2005-85904 A
[Patent document 2] JP 2004-119721 A
[Patent document 3] JP 2004-55974 A